The lymphoid system performs critical functions in animals that include preventing and combating infection, and surveillance and immune elimination of tumor cells. Loss of immune function leads to an immunocompromised status that can predispose the host to serious and life-threatening disease. Functional abnormalities may be present in any of the elements that participate in mediating an immune response, e.g., cellular or humoral elements such as granulocytes, lymphocytes, complement, antibody, or cytokines.
The immune system is a network of cells adapted to protect the organism against pathogens and cells that are not recognized as “self.” Once the immune system is activated, it enlists the participation of a variety of cells and molecules to mount an effector function designed to eliminate the “non-self” entity within the body. Lymphocytes are cells of the immune system that are capable of specifically recognizing and selectively eliminating foreign entities. By contrast to other cells of the immune system, such as neutrophils which are considered non-specific in their reactions to invaders, the characteristics of lymphocytes confer specificity, diversity, memory and self/nonself recognition to the immune response.
There are two major populations of lymphocytes: B lymphocytes and T lymphocytes. B lymphocytes originate and mature within the bone marrow and are responsible for formation of antibody molecules. T lymphocytes also arise from the bone marrow but mature in the thymus. There are two major subpopulations of T-cells: T helper cells and T cytotoxic cells. The two types of T cells can be distinguished by the presence of one of two membrane glycoproteins, either CD4 or CD8. The T-helper cells (which express CD4) when activated by antigen-complexes (foreign molecules coupled to special proteins) respond by secreting various growth factors known collectively as cytokines. These cytokines are signals that activate other cells of the immune system, including the T-cytotoxic cells. The T-cytotoxic cells (which express CD8) when activated, proliferate and differentiate into cytotoxic T lymphocytes (CTL) which are able to monitor for and eliminate from the body pathogenic cells, foreign cells, virus-infected cells, and tumor cells.
The normal development, maturation and differentiation of T lymphocytes are regulated by various peptide factors.